1. Field of the Invention
This invention relates to a new silacyclohexane compound, a method of preparing it, and a liquid crystal composition which contains it as well as a liquid crystal display element which contains said liquid crystal composition.
2. The Prior Art
A liquid crystal display element utilizes the optical anisotropy and dielectric anisotropy of liquid crystal substances. Display methods include the TN mode (twisted nematic mode), the STN mode (super twisted nematic mode), the SBE mode (super birefringence mode), the DS mode (dynamic scattering mode), the guest-host mode, the DAP mode ("deformation of aligned phase" mode) and the OMI mode (optical mode interference mode). The most common display device has a twisted nematic structure based on the Schadt-Helfrich mode.
The properties required of the liquid crystal substance used in these liquid crystal displays are somewhat different depending on the display method. However, a wide liquid crystal temperature range and stability against moisture, air, light, heat, electric fields, etc., are commonly required in all display methods. Furthermore, it is desirable for the liquid crystal material to have a low viscosity, and also to have a short address time, low threshold voltage and high contrast in the cell(s).
Currently, there is no single compound which satisfies all these requirements. In practice, liquid crystal mixtures are obtained by mixing several to more than ten liquid crystal compounds and latent liquid crystal compounds. Because of this, it is also important that components of a liquid crystal composition mix easily.
The components of the liquid crystal mixture described above can be classified into several categories based on their functions, as shown below:
1) Compounds which contribute to a reduction in viscosity and a lowering of the melting point of the mixed liquid crystal composition PA1 2) Compounds which mainly control the electro-optical functions of the mixed liquid crystal composition PA1 3) Compounds which contribute to raising the clearing point of the mixed liquid crystal composition PA1 4) Compounds which contribute to refraction anisotropy control of the mixed liquid crystal composition PA1 5) Compounds which control the colored display and orientation of the mixed liquid crystal composition
For compounds which belong to the category "1) Compounds which contribute to a reduction in viscosity and a lowering of the melting point of the mixed liquid crystal composition" in this classification, compounds with a so-called EPCH structure such as ##STR3## (Japanese examined patent publication (Tokko) Sho 59-35901; X denotes a halogen atom) and ##STR4## (Tokko Hei 3-46454) have been known.
In recent years, along with the expansion of the applications of liquid crystal displays, the characteristics required of liquid crystal materials are becoming more and more advanced and demanding. In particular, superior characteristics such as a lower driving voltage and improved low temperature performance, compared with conventional liquid crystal substances, are desired.